This invention relates a liquid crystal display device, and more particularly to a technique which is effectively applied to a circuit for supplying a video signal voltage to each pixel.
The active-matrix type liquid crystal display device having an active element for each pixel (for example, a thin film transistor) and switching the active elements has been used widely as a display device of a notebook personal computer.
TFT (Thin Film Transistor) type liquid crystal display module has been known as one of the active-matrix type liquid crystal display device. In a TFT type liquid crystal display module, since a video signal voltage (a gray scale voltage) is applied to a pixel electrode through a thin film transistor (TFT), the TFT type liquid crystal display module is free from crosstalk, and it is possible that the TFT type liquid crystal display module provides a multi-gray scale display without using a special driving method, unlike a simple matrix type liquid crystal display device, which requires a special driving method for preventing crosstalk between pixels.
As a method for applying a multi-gray scale video signal voltage to each pixel to render an active-matrix type liquid crystal display device capable of the multi-gray scale display, a method described in Japanese Published Unexamined Patent Application No. Hei 5-35200 (published on Feb. 12, 1999) (corresponding to U.S. Pat. No. 5,337,070, issued on Aug. 9, 1994) has been known.
Japanese Published Unexamined Patent Application No. Hei 5-35200 discloses a method in which 2m voltage bus lines are provided, and gray scale voltages provided from the 2m voltage bus lines vary in a staircase fashion having 2k steps during one horizontal scanning period corresponding to one horizontal scanning line.
One of the above-mentioned 2m voltage bus lines is selected based on the high-order m bits of an n-bit display data, one of the voltage levels is selected from the gray scale voltage varying in a staircase fashion on the selected voltage bus line based on the lower-order k (k=n-m) bits of the n-bit display data, and the selected voltage level is applied to a pixel electrode of each pixel.
For example, a case in which the display data is 3 bits (n=3), m=1, and k=2 is assumed. Two voltage bus lines are provided and each voltage bus line is supplied with a gray scale voltage varying in a staircase fashion having four steps during one horizontal scanning period such that eight voltage levels of the two gray scale voltages are different from each other.
A gray scale voltage carried on one of two voltage bus lines is selected based on the high-order 1 bit of the 3-bit display data, one voltage level is selected from the gray scale voltage varying in a staircase fashion having four steps on the selected voltage bus line, based on the lower-order 2 bits of the 3-bit display data, and the selected voltage level is applied to the pixel electrode of each pixel.
According to the driving method described in the above-mentioned publication, the operating speed of the circuit for applying a video signal voltage on each pixel can be reduced, and the number of voltage bus lines can be reduced.
Recently, the liquid crystal display device has been increased in the number of steps of the gray scales to 64 or 256.
In the case where gray scales of 64 or 256 steps is realized by the method described in Japanese Published Unexamined Patent Application No. Hei 5-35200, the circuit scale of a selector circuit for selecting voltage levels varying in a staircase fashion having 2k steps on the selected voltage bus lines should be large. In the case where the selector circuit is incorporated into a liquid crystal display panel, an area occupied by the selector circuit should be large, and the liquid crystal display panel should be consequently large. The large size is disadvantageous for the liquid crystal display panel.